MAN’S best friend comes in all shapes and sizes. From the Chihuahua to the Great Dane, the domestic dog is the most diverse mammal species of all, with over 350 distinct breeds for hunting, pulling sledges, tracking, herding and, of course, companionship.

Such diversity led Darwin to think that domestic dogs must have evolved from several wild canines, such as jackals, coyotes and wolves, but in fact we now know that all breeds descended from one species, the grey wolf. What remains a mystery is how the grey wolf’s DNA spawned so much canine variety.

Matthew Webster of Trinity College Dublin, Ireland, and colleagues at Uppsala University in Sweden believe they have an explanation, after analysing the mitochondrial DNA of both dogs and wolves. Mutations in the canine genome have played a bigger role in dog evolution than we thought, says Webster, and have given breeders a rich genetic palette to dip into (Genome Research, DOI: 10.1101/gr.5117706).

Mutations in the canine genome played a bigger role in dog evolution than thought” When grey wolves were domesticated around 15,000 years ago, the pressure of natural selection was relaxed, he explains. Natural selection usually weeds out a mutation in a wild species if it fails to offer a survival advantage. But if that pressure is removed, these mutations will pass down the generations, says Webster, especially where populations were small, which was likely early on in the dog’s domestication. This provided dog breeders with a wide inventory of traits in the DNA to exploit.

The full diversity of the canine genome was only truly unleashed a few hundred years ago when humans began artificially selecting dogs with desirable characteristics. For example, barking in the presence of intruders may have been selected for early in domestication, whereas later breeders would have gone on to artificially select for a Dalmatian’s spots or a pitbull’s aggression.

“Our findings highlight the importance of mutation in driving evolution,” says Webster. “With weaker natural selection, you can get a lot more changes in proteins that can be important in the future of the species.” Some mutations can have a greater impact because they influence the expression of lots of interacting genes. “It may not take a large number of mutations to generate a lot of diversity,” adds Webster.

Robert Wayne, an evolutionary biologist at John Hopkins University points out that the researchers have only studied mutations in mitochondrial DNA, which tends to accumulate more quickly than in nuclear DNA which forms the largest part of the genome. Far fewer mutations may have occurred in genes in the nucleus compared with mitochondria, he says. “Much of the variation we see in dogs may have to do with pre-existing variation from the ancestral wolf-dog population. Relaxation of selection is likely to be only part of the answer.”